There has been some recent online drama related to the doctrine of the Trinity. I can't say every voice is equally helpful, but there are some solid things to read on this critically important doctrine--- and it's all free. There is much mystery surrounding the tri-unity within the Godhead, so humility and caution should characterize us as wade into it. But, God has revealed this truth in his word, and at this stage in the history of the church there is no excuse for a Christian to remain ignorant of, or unfamiliar with, the doctrine of the Trinity. So, be sure to jump into all or some of these free resources.
R.C. Sproul, What is the Trinity? (eBook / PDF)
More, including free audio on the Trinity at Ligonier.